The present invention relates to optical devices that can be used on a motor vehicle for detection of the condition of the road surface, for example, for detecting whether the road surface is dry or wet and/or covered with snow and/or ice.
Devices of this type are already known in the art. For instance, the U.S. Pat. No. 5,497,100 describes a device that makes use of an emitter of electromagnetic radiation having frequencies comprised in the spectral band of microwaves and which analyses the components of the radiation reflected from the road surface, distinguishing whether said surface is dry, wet, or icy, on the basis of the different dielectric constants between these conditions, which affect the reflected radiation.
Other types of devices use the spectral band of the infrared, as described, for example, in the document DE-A-4008280. In this spectral range, in fact, there exist wavelengths that can be selectively absorbed by a wet or icy layer so that, by making the ratio between the signals reflected for the wavelengths of interest, it is possible to identify the road condition. However, with this methodology, even though it is relatively easy to distinguish between a condition of dry road and a condition of road that is not dry, the distinction between a wet condition and an icy condition proves to be more problematical. For this reason, for example in DE-A-4040842, two devices are combined that are able to detect the radiation reflected in the spectral bandwidths of the microwaves and of the infrared. Or else in US2004-204812 there has been proposed the use of temperature sensors, ultrasound sensors, and telecameras. The use of these methodologies is disadvantageous, above all for use on vehicles, on account of the high cost of the device.